1. Field of the Invention
The present invention relates to a switching power supply device of a half-wave rectification current resonance type. In particular, the present invention relates to a starting method (so-called soft start technique) to be applied when an input power source is turned on, or after the completion of an overcurrent protection operation.
2. Description of the Related Art
Conventionally, switching power supply devices have been used that are designed to receive power from a commercial alternating current power source and supply a predetermined rated level of direct current voltage to a load. In general, such devices are designed to gradually increase an output voltage in order to prevent an overvoltage and/or an overcurrent from occurring during a initial transient period after the application of power, which is generally called a soft start.
For example, a soft start method disclosed in Japanese Patent Application No. 2006-50688 (Patent Document 1) relies on the fact that the higher the switching frequency the lower the output voltage, and that the lower the switching frequency the higher the output voltage. This soft start method operates, if a switching power supply device is of a so-called wide range adaptive type, i.e., the device is able to receive power from a commercial alternating current power source of an AC100V type, an AC 200 V type, etc., so as to gradually decrease a switching frequency from a predetermined frequency in the process of a soft start operation, thereby decreasing at the rate corresponding to the voltage level of the commercial alternating current power source. This soft start method is thereby able to keep the duration of a soft start operation constant no matter what voltage level the commercial alternating current power source to be used provides.
A soft start method disclosed in Japanese Patent Application No. 2007-20327 (Patent Document 2) is designed to gradually lengthen the on period of the high-side switching element. Throughout the period in which this proceeds, the low-side switching element is made to remain off. Alternatively, the on period of the low-side switching element is initially made to be shorter than the off-duty period of the high-side switching element to subsequently be gradually lengthened. A soft start operation is thereby performed.
A soft start method disclosed in Japanese Patent Application No. 2007-189877 (Patent Document 3) is designed such that at the start operation when the direct current power source is turned on, or at the restart of switching after the completion of activation of an overcurrent protection circuit, the on period of the high-side switching element is gradually lengthened. A soft start operation is thereby performed.
FIG. 8 shows one example for a conventional switching power supply device. This switching power supply device is, as shown in FIG. 8, a current resonance type and is designed such that the secondary winding Ns is located on one side of the transformer T1, and on this side, a half-wave rectifying circuit is formed. A switching power supply device of a current resonance type designed in such a way is referred to as a switching power supply device of a half-wave current resonance type. As shown in FIG. 8, and the half-wave rectification current resonance type switching power supply device 100 will be described below.
A direct current power source Vin, a high-side switching element QH (first switching element), and a low-side switching element QL (second switching element) are connected in series. Reference symbols DH and DL each refer to a body diode connected to the high-side switching element QH and the low-side switching element QL in antiparallel, respectively. Also, a voltage resonant capacitor Crv is connected to the low-side switching element QL in parallel. Further, the low-side switching element QL has a serial resonant circuit connected thereto in parallel, the serial resonant circuit comprises a reactor Lri, a primary winding Np (exciting inductance Lp) of an output transformer T1, and a current resonant capacitor Cri. The secondary winding Ns of the transformer T1 is connected to a diode RC in series. The secondary winding Ns of the transformer T1 is also connected to a smoothing capacitor Co in series. Direct current power that has been smoothed by a smoothing capacitor Co is supplied to a load Ro. As a high-side switching element QH and a low-side switching element QL, MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor), for example, may be used.
FIG. 9 is a signal waveform diagram of the half-wave rectification current resonance type switching power supply device 100 shown in FIG. 8. The behavior will be described below.
First, the period in which the low-side switching element QL is off, and the high-side switching element QH is on, i.e., the period t1 to t2 in which the drain-source voltage VQL of the low-side switching element QL is at a high level will be described below.
In this period, first, from the direct current power source Vin, via the high-side switching element QH, the reactor Lri, the primary winding Np (exciting inductance Lp) of the output transformer T1, and the current resonant capacitor Cri, a resonant current ICri due to the reactor Lri, the primary winding Np (exciting inductance Lp) of the output transformer T1, and the current resonant capacitor Cri passes to charge current resonant capacitor Cri.
Then, the period in which the low-side switching element QL is on, and the high-side switching element QH is off, i.e., the period t2 to t3 in which the drain-source voltage VQL of the low-side switching element QL is at a low level will be described below.
In this period, first, the voltage across the current resonant capacitor Cri is applied to the transformer T1, thereby inverting the polarity of the voltage applied across the transformer T1, which in turn turns on the diode RC connected to the secondary winding of the transformer T1. Thus, in this case, the resonant current ICri is generated by the resonant circuit comprises the reactor Lri and the current resonant capacitor Cri. The resonant current ICri decreases due to discharge of the current resonant capacitor Cri so as to finally flow in the opposite direction, which causes energy to be transmitted to the secondary side of the transformer T1. On the secondary side of the transformer T1, the resonant current ICri charges the smoothing capacitor Co via the diode RC so that direct current power is supplied to the load Ro.
That is, the half-wave rectification current resonance type switching power supply device 100 is designed to continue to charge the current resonant capacitor Cri as long as the low-side switching element QL remains off and the high-side switching element QH remains on. Also, the half-wave rectification current resonance type switching power supply device 100 is also designed to continue to supply direct current power to the load by transmitting energy to the secondary side of the transformer T1 via discharge of the current resonant capacitor Cri as long as the low-side switching element QL remains on and the high-side switching element QH remains off. In addition, the switching elements QH and QL are prevented from being turned on at the same time by turning on the switching elements QH and QL alternately with a dead time in between. Also, the voltage resonant capacitor Crv serves to subject the high-side and low-side switching elements QH and QL to a voltage resonance when the high-side and low-side switching elements QH and QL are turned on or off, respectively.
An amount of energy to be transmitted to the secondary side of the transformer T1 is determined by an amount of charge of the current resonant capacitor Cri. Therefore, an amount of energy to be transmitted to the secondary side of the transformer T1 is able to be controlled by varying the on period (a period from t1 to t2) of the high-side switching elements QH.
Resonant current generated by the resonant circuit comprises the capacitor Cri and the reactor Lri serves to transmit energy to the secondary side of the transformer T1. Therefore, the period in which energy is transmitted to the secondary side of the transformer T1 is constant. The on period of the low-side switching element QL may be fixed.
Therefore, an amount of energy to be transmitted to the secondary side of the transformer T1 is controlled via frequency control designed to make the on period of the high-side switching elements QH variable and also designed to fix the on period (t2 to t3) of the low-side switching element QL. When the on period of the high-side switching elements QH is shortened, this causes an amount of charge of the capacitor Cri to be decreased. This in turn causes an amount of energy to be transmitter to the secondary side of the transformer T1 to be decreased. Therefore, at the time of starting, the on period of the low-side switching element QL is fixed, and the on period of the high-side switching elements QH is gradually lengthened. A soft start is thereby performed.    Patent Document 1 refers to Japanese Patent Application No. 2006-50688.    Patent Document 2 refers to Japanese Patent Application No. 2007-20327.    Patent Document 3 refers to Japanese Patent Application No. 2007-189877.